Gardevoir can do many things. It can use Life Dew to heal itself and a teammate, it can set Trick Room, burn with Will-O-Wisp, set up dual screens, scout for abilities with Trace, hit both enemy Pokemon with STAB Dazzling Gleam, power-up allies with Helping Hand, and use Icy Wind to slow down enemies, if you aren't running Trick Room. It also can dodge things like Sludge Wave, Earthquake, and Surf due to its ability Telepathy. In conclusion, Gardevoir is a lesser-seen but decent choice as a support Pokemon, and can do quite a bit of damage itself while scouting abilities.
Hope this helped!